Hypodermic syringe with passive aspiration feature

ABSTRACT

The invention comprises a disposable hypodermic syringe with means for passive aspiration. The syringe comprises a barrel ( 10 ) of clear synthetic plastics material and a plunger ( 12 ) comprising an elastic bung ( 28 ) slideable in the barrel ( 10 ). Pressure on the plunger elastically deforms the bung ( 28 ). If this pressure is released the bung ( 28 ) resiles to effect aspiration. This aspiration is passive, requiring no active withdrawal of the plunger by the user. After use the syringe may be disposed of as a whole, with no disassembly, using one hand only. The syringe may be made by adapting cartridges conventionally used for dental local anaesthetic.

[0001] This invention concerns hypodermic syringes and particularly thesafety thereof.

[0002] The safety of hypodermic syringes as addressed herein fallsbroadly into two categories, that is patient safety and practitionersafety, and these two matters will now be considered in turn.

[0003] When using a hypodermic syringe it is generally necessary forreasons of patient safety to check the position of the needle tiprelative to blood vessels to ensure that potentially toxic material isnot injected into the circulatory system. Intravascular injection givesrise to the risk of a toxic reaction, especially if administeredquickly, when the injectant may travel around the body as a bolus toarrive at the brain in abnormally high concentration. This may happenwith any injectant but is notably serious in the case of localanaesthesia, especially if the anaesthetic agent is combined withadrenaline, since adrenaline entering the circulation system may havefatal consequences. The danger is particularly severe in dental localanaesthesia because the injection site is so close to the patient'sbrain.

[0004] The standard method of checking the position of the needle tiprelative to blood vessels is by a procedure known as aspiration. Afterthe practitioner has inserted the needle, the plunger of the syringe isretracted by a small amount to obtain a sample of the surrounding bodymaterial If the tip of the needle is in a blood vessel, a sample ofblood will be drawn back into the syringe where it can be observed.Then, if blood is seen, the practitioner can begin again, altering theposition of the needle. To allow for movement of the needle, aspirationmay need to be repeated one or more times during the course of theinjection.

[0005] There are two kinds of aspiration, active and passive. Activeaspiration is a procedure in which the practitioner retracts the plungermanually. Active aspiration is somewhat awkward and may lead to movementof the needle, as well as of the plunger, making the aspirationuncertain. Passive aspiration (sometimes called “self-aspiration” whichfor the purposes of the present application is deemed to mean the samething) is much safer. Passive aspiration is aspiration that takes placeautomatically when pressure on the plunger of the syringe is released,rather than the plunger having to be actively retracted. In other words,providing a syringe with means for passive aspiration means that thepractitioner does not have to move the plunger manually to carry out theaspiration procedure. And this ensures that the injection site will bethe same as the sampling site.

[0006] It is an object of the present invention to improve patientsafety by providing a hypodermic syringe with passive aspiration.

[0007] The essential sharpness of hypodermic needles inevitably meansthat users occasionally injure themselves. Until comparatively recently,these so-called needlestick injuries were almost always no more thanpainful, and regarded as an occupational hazard by doctors, dentists andso forth. However perceptions have been changed by the spread of seriousdiseases such as auto-immune deficiency syndrome (AIDS) and hepatitisand the resurgence of tuberculosis in developed countries. Such diseasesare readily transferred by needle and needlestick injuries are nowperceived as potentially life-threatening. It will be understood thataspiration amplifies the threat by inducting a much greater amount ofpotentially infected material into the syringe.

[0008] The prevalence of needlestick injuries is indicated by a tradeunion estimate of twenty thousand such injuries a year in Scotlandalone. The seriousness is reflected in data from UK occupational healthdepartments showing eight hundred and thirteen reports of exposure tobloodborne viruses at work between July 1997 and June 2000. (And atleast one practitioner caught HIV despite immediate prophylaxis).Official concern is marked by the Needlestick Safety and Prevention Actpassed (unanimously) by the US Congress and signed into force inNovember 2000.

[0009] In dentistry it has been universal practice over many years toemploy hypodermic syringes with needles that after use are resheathed,detached and placed in a container known as a sharps box for disposal.However the risk of a needlestick injury is present during the periodbetween completing the injection and placing the needle in the sharpsbox. The risk is obviously greater if this period is extended—as when apractitioner puts a syringe aside after use (perhaps, understandably, toattend to the patient) instead of dealing with its disposal straightaway. It is also, of course, not uncommon for a practitioner to compoundthe risk by passing the used syringe to a nurse or other assistant.

[0010] However the most acute, and avoidable, risk stems frompost-injection disassembly, and in particular resheathing the needle.(Notably, regulations in USA prohibit resheathing of hypodermic needlesbut dentistry has a specific exemption). Disposability of the syringe asa whole will make it quick and easy for the practitioner to dispose ofit immediately, and the needlestick risk will be substantially reduced.(The use of a disposable syringe in dentistry will also, of course,remove the need for the dental exemption).

[0011] It is therefore another object of the invention to improvepractitioner safety by providing a hypodermic syringe which isdisposable after use, that is as a wholes without the need to detach theneedle or any other part.

[0012] At this point may be noted another aspect of patient safety whichis particularly relevant to dental local anaesthesia. Dental surgeonsinvariably use a cartridge type of syringe which has a reusableactuating mechanism. A cartridge containing local anaestheticagent—typically an amino-ester or an amino-amide mixed with a proportionof adrenaline—is loaded into a chamber of the actuating mechanism. Thecartridge has a slideable bung arranged to be engaged by a plunger ofthe actuating mechanism when the cartridge is in the chamber. After thecartridge has been loaded into the chamber a needle is attached to itsforward end, usually by means of a screw thread arrangement. Then thepractitioner can depress the plunger to drive the bung into thecartridge and expel the contents thereof to make an injection in theusual way. After use, the cartridge and needle are disassembled anddisposed of, and the actuating mechanism sterilised for reuse.

[0013] The problem in this, as will be appreciated by thoseknowledgeable in the science, is that certain disease vectors—such asthe prion proteins associated with variant Creutzfeldt Jakob disease(VCJD)—can survive conventional sterilisation. It is therefore anotherobject of the invention to provide a hypodermic syringe which does notrequire sterilisation after use.

[0014] A disposable syringe is the only effective safeguard againstcross-contamination by way of the syringe, but the dental profession hasnot hitherto used disposable syringes for local anaesthesia. It will beappreciated, however, that a disposable syringe can go entirely into thesharps box waste container straight after use. And importantly, thedentist can do this using just the (one) hand holding the syringe,without exposing his/her other hand, any other part of his body or anyancillary (or even, as may be the case, another patient) to needlestickrisk.

[0015] For the avoidance of doubt it should be observed that disposablesyringes (which of course do not entail post-injection sterilisation)have been widely used over many years in medical and veterinarypractice, but not in dentistry. By contrast, passive aspiration is wellestablished in dentistry but not elsewhere. It is therefore a finalobject of the present invention to combine the benefits of disposabilityand passive aspiration to the advantage of all medical, dental andveterinary practice.

[0016] Certain previously known syringes will now be discussed.

[0017] In dental local anaesthesia the toxicity problem noted earlierhas made pre-injection aspiration a standard recommendation—see, forinstance, “Introduction to Dental Local Anaesthesia'” Mediglobe SA,1990- and it was in this field that passive aspiration was firstintroduced, by Evers, more than thirty years ago. In U.S. Pat. No.3,295,525 Evers et al disclose a hypodermic syringe of the cartridgetype which is arranged to be automatically self-aspirating. Thecartridge is provided with a bung having a bore extending through it toa resilient diaphragm at the head of the bung (that is, that part of thebung which engages the injectant in the cartridge). A specially designednose on the end of the plunger in use extends through this bore toengage the diaphragm. When the plunger of the actuating mechanism ispressed the diaphragm is deflected inwardly and when the plunger isreleased the diaphragm automatically returns to its normal position,creating a pressure reduction within the cartridge to effect the desiredaspiration. If aspiration is satisfactory the injection may proceed, thepractitioner pressing the plunger further forward to cause a flangebehind the nose to engage the bung and drive it into the cartridge andexpel the injectant.

[0018] The Evers arrangement has a number of disadvantages. First, therequirement for a diaphragm complicates the manufacture of the bung.Second, the actuating mechanism has to be specially made or modified toprovide the nose and the flange described above. Third, the specialself-aspirating cartridge may only be used with this form of syringe.Fourth, the special syringe cannot be satisfactorily used with astandard cartridge because, without a bore in the bung, the extensionmay make the bung tip over and leak. And finally, the actuatingmechanism is not disposable and, as noted earlier, its reuse requiressterilisation that is likely to be less than perfect.

[0019] In U.S. Pat. No. 4,216,771 Arlers et al disclose a hypodermicsyringe with aspiration which is specifically stated to be a disposablearticle. However, this syringe does not provide passive aspiration. Ithas a plunger comprising circumferentially inner and outer partsconnected together by a diaphragm at the head of the plunger. Anaspiration rod is connected to the inner part and extends through alongitudinal channel in the plunger to an actuating member pivotallymounted at the outer end of the plunger. The practitioner effectsaspiration by manually pivoting the actuating member to draw the innerpart of the plunger backwards.

[0020] Aside from its complexity, with a number and variety of movingparts, the Arlers syringe offers active aspiration rather than passiveaspiration. It therefore fails to secure the benefits of passiveaspiration noted hereinbefore. In fact U.S. Pat. No. 4,216,771 pointsaway from passive aspiration, criticising one previous arrangement(apparently the Evers syringe) as being difficult and awkward to use andanother as being complicated and expensive. The present inventionenables both these criticisms to be obviated.

[0021] An alternative to Evers's arrangement for passive aspiration, andmore recent, is shown in U.S. Pat. No. 5,411,488 to Pagay et al. Thisdiscloses a syringe barrel pre-filled with injectant and closed by aslideable bung (or “plunger”) of elastomeric material. The barrel hasmeans at one end for attachment of a needle before making an injection.A plunger rod has a convex tip which fits into a blind bore in the bungand before use is connected thereto by a collar captive on the tip andformed with a screw thread for connection to the bung. The collar haslimited sliding movement with respect to the plunger rod so that whenthe latter is depressed it first of all moves forward against the end ofthe blind bore to distort the head of the bung inwardly. If pressure onthe plunger rod is then released the head of the bung returns to itsnormal shape, thereby effecting aspiration. Subject to satisfactoryaspiration the plunger rod may then be depressed further, when ashoulder on the rod engages the collar screwed into the bung to drivethe bung into the barrel and eject its contents.

[0022] Those skilled in the science will appreciate that Evers and Pagayprovide aspiration in substantially the same way: in Pagay the convextip of the plunger deforms the head of the bung just as in Evers theextended nose the plunger acts on the elastic diaphragm of the bung. Itfollows that Pagay's arrangement shares a number of the disadvantagesnoted above in relation to Evers. The provision of a screw thread inPagay makes the bung even more complex. Similarly Pagay's plunger iscomplicated, and it requires the practitioner to connect it to the bungbefore an injection can be made. There is no mention of disposability inU.S. Pat. No. 5,411,488, but the very complexity of the plunger showntherein (and by inference its cost) suggests that practitioners would beencouraged to reuse it; and the persistence of certain disease vectorsnoted earlier makes this hazardous even with sterilisation.Significantly, thirty years elapsed between Evers and Pagay but therewas no move towards combining disposability with passive aspiration inall that time, nor has there been in the years which have elapsed sincePagay.

[0023] According to the present invention there is provided a disposablehypodermic syringe including means for passive aspiration.

[0024] The term “disposable” is used herein to refer to a hypodermicsyringe which is designed and made to be used once only and thendisposed of as a whole, with no disassembly and no part retained. (Aquasi-judicial definition of the term “disposable” may be derived fromstatutory regulations such as those of the European Union. For instance,the Medical Devices Directive, which is legally binding on all EU memberstates, requires that all medical devices be supplied with instructionsas to their assembly, use and sterilisation or safe disposal, so thatany medical device which is non-disposable must be accompanied bysterilisation instructions. It follows that a disposable hypodermicsyringe is one that is classified as disposable under the MedicalDevices Directive and supplied with disposability instructions.) Suchdisposability may be carried out by a practitioner using just one hand,with negligible risk of needlestick injury. The single use obviatessterilisation which is likely to be imperfect and may allowcross-contamination over a long time period. And passive aspiration is asafe and sure way of checking that the syringe is properly positioned inuse. Thus the invention fulfils a long felt want in the practice ofhypodermic injections by combining disposability with passiveaspiration.

[0025] A disposable hypodermic syringe according to the inventionpreferably includes a barrel for injectant and a plunger depressibleinto the barrel to drive the injectant therefrom and is arranged so thatdepression of the plunger causes elastic deformation of part of theplunger within the barrel, whereby passive aspiration is provided.

[0026] The plunger may comprise a resilient bung in the barrel and arelatively rigid stem extending outwardly therefrom. The stem and thebung are preferably permanently connected together, and may be ofunitary construction, possibly of similar elastomeric material (such asnatural rubber or a synthetic plastics material) differentially treatedto make the stem relatively rigid and the bung relatively elastic.

[0027] The bung may be formed with an elastically deformable diaphragm,to which the stem of the plunger may be attached. There may be a vent,possibly including a cavity, between the diaphragm and the interior ofthe syringe. Such a syringe may be arranged so that pressure on theplunger causes elastic deformation of the diaphragm towards the interiorof the barrel and releasing said pressure allows the bung to resile andeffect the required passive aspiration. The respective moduli ofelasticity of the stem and the bung may be adapted to adjust therelative movement of the stem and the bung when the plunger is pressed.

[0028] Preferably the barrel of the syringe is transparent, and it maycomprise a tube of clear synthetic plastics material. The barrel maycontain a metered amount of injectant such as a local anaesthetic agentand be labeled to indicate the injectant and the amount.

[0029] A syringe according to the invention may comprise a seal closingthe barrel, a hollow needle and connecting means such as a Luer lock orother screw thread for connecting the needle to the barrel in a firstposition, wherein the connecting means is adjustable to cause or permitthe needle to move to a second position in which it pierces said seal.With this arrangement the needle may have a tip at one end forhypodermic intromission and a tail at the other for piercing said sea),and preferably the barrel, plunger, seal, needle and connecting meansare after use disposable connected together.

[0030] A syringe according to the invention may alternatively have aneedle permanently connected to its barrel.

[0031] A disposable hypodermic syringe according to the invention may bemade by adapting a conventional cartridge for dental local anaesthetic,with only moderate modification of existing manufacturing plant, bysecuring to the forward end of said cartridge connection means for ahypodermic needle and securing to the rearward end of the cartridge anassembly comprising a slideable bung providing means for passiveaspiration, a stem for sliding the bung and a cover providing a guidewayfor the stem. The connection means may be secured to the cartridge by acap which also secures the conventional seal thereto.

[0032] The invention extends to apparatus for administering aninjection, which apparatus comprises a sterile pack containing ahypodermic syringe according to the invention and prescribinginformation as to Its use.

[0033] The invention also comprehends a method of guarding againstcross-contamination in a hypodermic injection, which method comprisesproviding a hypodermic syringe with means for passive aspiration, usingthe means for passive aspiration to aspirate before performing theinjection and disposing of the whole syringe immediately afterperforming the injection.

[0034] Other features of the invention will be apparent from thefollowing description, which is made by way of example only and withreference to the accompanying schematic drawings to an enlarged scale inwhich—

[0035]FIG. 1 is a cross section of a disposable hypodermic syringeincluding means for passive aspiration according to the invention;

[0036]FIG. 2 is a partial view showing the effect of applying pressureto the bung of the syringe shown in FIG. 1;

[0037]FIG. 3 is a partial view showing the effect of releasing pressurefrom the bung of the syringe shown in FIG. 1;

[0038] FIGS. 4 to 7 are partial views illustrating other means forpassive aspiration in disposable syringes according to the invention;

[0039]FIGS. 8 and 9 show in cross section means for attaching a needleof a disposable hypodermic syringe according to the invention;

[0040] FIGS. 10 to 12 illustrate the adaptation of a conventionalcartridge for dental anaesthetic to produce a disposable hypodermicsyringe according to the invention.

[0041]FIG. 1

[0042] Referring first to FIG. 1, a syringe comprises a barrel indicatedat 10, a plunger indicated at 12 and a needle assembly indicated at 14.

[0043] The barrel 10 comprises a transparent tube 16 made of syntheticsplastics material. The tube may contain a local anaesthetic agent orother medium to be injected, or it may alternatively be empty, eg todraw off a blood sample.

[0044] At one end the tube 16 is formed with a finger grip 20 extendinglaterally on both sides. The needle assembly 14 is at the other end ofthe barrel 10. It comprises a fine hollow needle 22 with its bore opento the interior of the barrel 10. The needle 22 is secured to the barrelin well5 known fashion by a moulded collar 24.

[0045] The plunger 12 is of cruciform section with webs 25 extendingradially outwards from a central stem 26 to provide a sliding fit forthe plunger 12 within the bore of the tube 16. The stem 26 extendsaxially beyond the webs 25 and into a bung 28. The stem 26 is made ofsynthetic plastics material and is relatively rigid. The bung 28 is madeof elastomeric material and is moulded on to the stem 26 to bepermanently connected thereto. (The bung 28 may be fixed to the stem 26by other means such as an adhesive; or it may be made of the samematerial as the stem and unitary therewith but differentially treated toalter its stiffness). The elasticity of the bung 28 allows the stem 26slight axial movement relative to the bung 28 and against elasticdeformation thereof. The bung 28 is a sliding fit within the tube 16 andits periphery sealingly engages the interior wall of the tube 16.(Although not shown, the bung 28 may be formed with peripheral rings orgrooves to reduce sliding friction with the tube 16 whilst 20 preservingthe seal therewith). The plunger 12 includes an integrally mouldedthumbpiece 30 at its outer end.

[0046]FIGS. 2 and 3

[0047] To use the syringe a practitioner holds it, using one hand only,with two fingers curled around the laterally extending finger grip 20and the thumb of the same hand on the thumbpiece 30, and inserts theneedle 24 into the target area. Referring now to FIG. 2, pressure on thethumbpiece 30 urges the stem 26 in the direction of arrow A. Before thebung 28 moves as a whole (that is, before thumb pressure overcomes thefriction between the bung 28 and the tube 16) the relatively rigid stem26 is displaced in a forward direction, distorting the bung 28elastically. This creates a swelling 32 at the inner face of the bung28. If the pressure on the thumbpiece 30 is now released, the bung 28resiles elastically to its original shape, as shown in FIG. 3. Theswelling at the inner face of the bung 28 subsides as indicated by arrowB. This creates a slight negative pressure within the tube 16, foraspiration.

[0048] It will now be understood that if the needle tip is in a bloodvessel, this negative pressure will draw a sample of blood into the tube16 where it can be seen by the practitioner. If it is intended that theneedle tip be in a blood vessel the practitioner can continue; otherwisehe can reposition the needle.

[0049] It will be noted that the elasticity of the bung 28 providespassive aspiration. In other words, by means of the invention aspirationtakes place automatically when pressure on the thumbpiece 30 isreleased. There is no need to withdraw the plunger 12 manually.

[0050] After the injection is complete, the practitioner (still usingjust the one hand holding the syringe) can dispose of the whole syringestraight away by simply depositing it in a sharps box. There is no needfor disassembly, and the procedure is so immediate that the practitionerwill not be inclined to put the used syringe aside or pass it on to anassistant. Thus disposal is quick and easy and above all safe. And as nopart of the syringe is retained for reuse there is no possibility ofcarry-over infection through inadequate sterilisation.

[0051]FIG. 4

[0052]FIG. 4 shows another way of providing passive aspiration. Anelastic bung 40 is slideable within a transparent tube 42 of syntheticplastics material. A sleeve 44 is interposed between the bung 40 and thestem 46 of a plunger 48. The sleeve 44 facilitates axial movement of thestem 46 relative to the bung 40, so that the stem 46 can be displacedwith less pressure, and also strengthens the sealing engagement of thebung 40 with the tube 42. This means in turn that the friction betweenthe bung 40 and the tube 42 can be reduced (eg by peripheralrings/grooves, the use of a softer elastomer and/or lubricant betweenthe body and the tube), to make the injection movement easier andtherefore smoother.

[0053]FIG. 5

[0054]FIG. 5 shows another way of making the injection movement easier.The stem 50 of a plunger 52 has a tapered enlargement 54 at its innerend that fits within a correspondingly shaped hole in an elastic bung56. When the plunger 52 is urged into a transparent tube 58 by thumbpressure the interaction of the tapered enlargement 54 with the bung 56it tends to contract the bung 56 so as to lessen its grip with the tube58.

[0055]FIG. 6

[0056]FIG. 6 shows a modification of the design of FIG. 5. The syringeshown partially in FIG. 6 has a stem 60 with its inner end formed with asubstantially spherical enlargement 62. The spherical enlargement fitswithin a correspondingly shaped hole in a bung 64 and the bung 64 is asliding fit in a transparent tube 66.

[0057] As described with reference to FIGS. 1 to 6 the inventionprovides passive aspiration in a disposable hypodermic syringe throughthe elastic deformation and recoil of the head of the bung of thesyringe (ie the face of the bung against the local anaesthetic agentwithin the syringe). This is not essential—indeed, the inventioncomprehends all kinds of passive aspiration in a disposable syringe—andanother means of passive aspiration is shown in FIG. 7.

[0058]FIG. 7

[0059] Referring to FIG. 7, a bung 70 of natural rubber or otherelastomeric material is slideable in a transparent tube 72 of syntheticplastics material. The bung 70 is formed on its outer side with adiaphragm 74 connected to the proximal end of the stem 76 of a plunger78. (The stem 76 protrudes somewhat beyond the cruciform section of theplunger 78 to engage the diaphragm approximately centrally). On theinner side of the diaphragm 74 are a cavity 78 and a passage 82extending to the inner face of the bung 70. Together the cavity 80 andpassage 82 make up a vent between the diaphragm 74 and the interior ofthe tube 72, which contains an injectant not shown. When the plunger 78is urged forward by thumb pressure the end of the stem 76 deforms thediaphragm 74 elastically inwards into the cavity 80 to reduce the volumethereof. It will be understood that the amount of flexing of thediaphragm 74 and thereby the change in volume of the cavity 80 dependsupon the dimensions and the modulus of elasticity of the diaphragm 74.If pressure on the plunger 78 is now released, the diaphragm 74 recoilselastically to its original shape. Thus the cavity 80 opens up againand, by way of the passage 82, creates a slight negative pressure withinthe tube 72. This results in the aspiration required, which will beunderstood to be passive aspiration. Subject to satisfactory aspiration,the plunger 78 may now be depressed again to slide the bung 70 forwardsalong the tube 72 to inject the local anaesthetic agent. The diaphragm74 is flexed again and remains flexed during the course of theinjection, so aspiration may be repeated at any time by releasing thepressure on the plunger 78.

[0060] The arrangement shown in FIG. 7 may be modified to have a form ofcavity different from that shown in FIG. 7. The cavity may for instancebe substantially spherical. In the simplest form the cavity and passagemay be merged so that the vent comprises a blind bore of substantiallyconstant cross section extending from the diaphragm to the inside of thesyringe.

[0061]FIGS. 8 and 9

[0062] Referring now to FIGS. 8 and 9, these use the same referencenumerals and are most conveniently considered together. The hypodermicsyringe shown in the figures comprises a barrel indicated at 110, aplunger indicated at 112 and a needle assembly indicated at 114.

[0063] The barrel 110 comprises a transparent tube 116 made ofsynthetics plastics material. The tube 116 contains a measured amount ofinjectant 118 (such as, for instance, 1.8 ml of a local anaestheticagent for use in dentistry). At one end the tube 116 is formed with afinger grip 120 extending laterally on both sides. At its other end thetube 116 is closed by a rubber seal (sometimes called a septum) 122 heldin place by a cap 124 secured to the tube 116 in well known manner. Thecap 124 is formed with a screw-threaded stud 126 having a central bore128.

[0064] The needle assembly 114 comprises a fine hollow needle 130secured in a collar 132. The needle 130 has a tip 130 a and a tail 130b, both pointed. The collar 132 has an internal screw-thread matchingthat of the stud 126, whereby the needle assembly 114 is connected tothe barrel 110.

[0065] As shown in FIG. 8, the collar 132 is partly screwed on to thestud 126. The needle 130 is then in a first position with its tail clearof the seal 122. It will be appreciated that the syringe may betransported as assembled in FIG. 8 because the seal 122 prevents leakageof the injectant 118 and/or prolonged contact with the needle 130 whichmay give rise to an adverse chemical reaction. Thus assembled a syringemay be supplied to a practitioner, in a sterile pack, prepared for use.Alternatively the syringe may be supplied separated from the needleassembly 114, which merely requires the practitioner to screw the collar132 on to the stud 126. (Provided the components are supplied in acommon sterile pack, the syringe qualifies for theatre use; and thoseskilled in the science will appreciate that it also avoids the risk ofan inappropriate needle being fitted and an injection being administeredto the wrong site with possibly catastrophic consequences).

[0066] To use the syringe, the practitioner screws the collar 132 homeon the stud 126 so that, as shown in FIG. 9, the tail 130 b of theneedle 130 pierces the seal 122. In this second position of the needle130, the plunger 112 may be pressed down to administer the injection 118by way of the needle tip 130 a.

[0067] It will be noted that the syringe shown in FIGS. 8 and 9 includesmeans for passive aspiration. The plunger 112 has a stem 134 extendinginto and secured to an elastic bung 136 slideable within the barrel 110.When the plunger 112 is pressed down, the stem 134 first of all slightlydilates the inner face of the bung 136. If the pressure is thenreleased, this inner face resiles to effect aspiration. If the needletip 130 a is in a blood vessel, this negative pressure will draw bloodinto the tube 116 where it can be seen by the practitioner. If it isintended that the needle tip be in a blood vessel the practitioner cancontinue; otherwise he or she can reposition the needle.

[0068] When the procedure has been completed the whole syringe isdisposed of, one-handed, simply by dropping it into an adjacent sharpsbox. Neither the practitioner nor any assistant is exposed to thepotentially contaminated needle tip 130 a. And sterilization-resistantvectors, as for VCJD, are overcome because the syringe is not usedagain.

[0069] Those skilled in the art will appreciate that the inventionoffers certain benefits additional to those hereinbefore described. Forinstance, having the stem 134 attached to the bung 136 means that ifrequired the plunger 112 can be positively retracted, for instance todraw in solvent to mix with injection in powder form. Having the syringepre-loaded with injectant allows the contents etc to be specified on thebarrel 110, along with any warnings about use, which is much safer thanmarking merely the packaging, especially for dangerous drugs orinjections requiring special precautions.

[0070] An important regulatory point may be noted here. Regulationsgenerally require any drug to be supplied with prescribing informationwhich must include information as to its disposal where appropriate. Ineffect, then, any syringe to be supplied pre-loaded with a drug must bedisposable.

[0071] Various modifications to the syringe of FIGS. 8 and 9 may be madewithout departing from the essence of the invention. The tube 116 may bemade of glass. The seal or septum 122 may be made of some material otherthan rubber. The cap 124 and the collar 132 may be made from light metalor synthetic plastics material and may be secured (respectively to thetube 116 and the needle 130) by bonding, crimping, heat-shrinking orotherwise. Further, the stud and collar shown in the drawing may bereplaced by a friction fit connector, or by a Luer lock having a hollowcollar and a rim fitting into an internal screw thread of a cap whichhas a tapered nose gripped by the hollow collar as it is screwed home.

[0072] As noted earlier herein, dental surgeons use a cartridge systemfor administering local anaesthetics, and it follows that substantialmanufacturing capacity is currently devoted to the production ofcartridges. Whilst the present invention is among other things directedtowards replacing the use of cartridges in dentistry by disposablesyringes, it will be appreciated that it would not be sensible—eithereconomically or logistically—to effect an instantaneous change. Howeverit is possible for existing manufacturing plant to be progressivelyturned over to disposable syringes according to the invention, and thisis illustrated in FIGS. 10 to 12.

[0073]FIG. 10

[0074]FIG. 10 shows, to a much enlarged scale, the forward part of aconventional cartridge for dental local anaesthetic. This cartridge isformed with a neck 200 (which is exaggerated in the drawing for the sakeof clarity). The end of the cartridge is formed with a hole 202 coveredby a seal or septum 204 keeping the anaesthetic secure anduncontaminated within the cartridge before use. (The other end of thecartridge is closed by a slideable bung, not shown). A metal cap 206 isswaged into the neck 200 or otherwise secured thereto to hold the seal204 in place. The cap 206 is formed with a hole 208.

[0075] The use of cartridges is well known and does not require adetailed description here. Briefly, the cartridge is loaded into thechamber of an actuating mechanism (sometimes loosely called a syringe)having a plunger whereby the bung can be driven into the cartridge. Ahypodermic needle is screwed on to the forward end of the actuatingmechanism to pierce the seal 204 by way of the hole 208 in the cap 206,after which the system is ready for use. When the injection has beencompleted, the needle and the cartridge have to be detached and disposedof, and the actuating mechanism sterilised.

[0076]FIG. 11

[0077]FIG. 11 shows a first adaptation of a conventional cartridge toprovide a disposable hypodermic syringe according to the invention. Thecartridge 220 of FIG. 11 is of the form described above in relation toFIG. 10 and has at its forward end a seal 212 held in place by a metalcap 214. An externally threaded stud 216 formed with a through bore 218is secured to the cartridge 210 by an integral skirt 220 swaged over thecap 214. The stud 216 is designed to receive a needle provided with aninternally threaded collar, not shown, as hereinbefore described withreference to FIGS. 8 and 9, and it may conveniently be formed as a Luerlock.

[0078] At the other end of the cartridge 210 is mounted a plungerassembly comprising a slideable bung 222, a stem 224 extending therefromto a thumbpiece 226 and a cover 228 formed with laterally extendingfinger grips 230. The cover 228 fits on to the rearward end of thecartridge 210 and is formed to provide a central guideway 232 for thestem 224. The bung 222, which is moulded on or otherwise secured to thestem 224, provides means for passive aspiration in the manner previouslydescribed herein.

[0079] The syringe of FIG. 11 can be made with little modification ofexisting plant for manufacturing the conventional cartridge. After theseal 212 and cap 214 have been applied to the cartridge 210 it isnecessary only to attach the threaded stud 216 before filling thecartridge with a metered amount of local anaesthetic in the usual wayand then add the assembly of bung 222, stem 224 and cover 228. (Twobenefits of the cartridge system are that because the cartridge ispreloaded in controlled conditions (a) there is no risk ofcross-contamination from the filling procedure and (b) the contents areassured as to both quantity and quality, avoiding problems in this areawhen syringes supplied empty are filled locally by practitioners; andpre-loading syringes according to the present invention offers identicalbenefits).

[0080]FIG. 12

[0081]FIG. 12 shows a further adaptation of a conventional cartridgewhich is simpler but requires more modification of existing plant. Inthe arrangement of FIG. 12, a metal cap 240 is used to secure both theseal 242 and the threaded stud 244 to the cartridge 246. Compared withthe arrangement of FIG. 11, this requires the stud 244 to be fitted onestep earlier in the manufacturing process and the use of a cap 240somewhat different and larger than the cap 214. In other respect thesyringe of FIG. 12 is similar to that of FIG. 11.

[0082] The adaptation of a conventional cartridge may have a significantregulatory advantage. Where the cartridge is already approved for use,for instance under the CE system of the European Union, the additionsdescribed above in relation to FIGS. 11 and 12 need not be submitted toprolonged and expensive approvals procedures because these additions arenon-invasive and therefore subject to self-certification by thesupplier.

[0083] Those knowledgeable in the practice of injections will appreciatethat the invention offers a simple and inexpensive hypodermic syringewith the benefits of disposability and passive aspiration. A syringeaccording to the invention is safe to use because, broadly speaking,disposability protects the practitioner and passive aspiration protectsthe patient—and this combination of features satisfies regulators andinsurers. Further, syringes according to the invention can readily bemade by adaptation of conventional cartridges, with commercial andregulatory benefits.

1. A hypodermic syringe comprising a barrel containing injectant,connecting means fixed to a forward end of the barrel for connecting ahypodermic needle thereto, a bung closing the rearward end of the barreland slidable into the barrel in use to displace the injectant therefromby way of the needle, and a plunger stem connected to the bung andextending rearwardly therefrom to be manually pressed to slide the bunginto the barrel, which hypodermic syringe includes means for effectingpassive aspiration in use and is disposable as a whole after use.
 2. Ahypodermic syringe as claimed in claim 1, wherein the means foreffecting passive aspiration comprises a portion of the bung which iselastically deformed when the plunger stem is pressed.
 3. A hypodermicsyringe as claimed in claim 2, wherein said portion of the bungcomprises an elastically deformable diaphragm.
 4. A hypodermic syringeas claimed in claim 3, wherein the bung is formed with a vent betweenthe diaphragm and the interior of the barrel.
 5. A hypodermic syringe asclaimed in claim 4, wherein the vent includes a cavity adjacent thediaphragm.
 6. A hypodermic syringe as claimed in claim 5, wherein thestem is attached to the diaphragm.
 7. A hypodermic syringe as claimed inany of claims 3 to 6, wherein pressure on the plunger causes elasticdeformation of the diaphragm towards the interior of the barrel andreleasing said pressure allows the diaphragm to resile and effect theaspiration.
 8. A hypodermic syringe as claimed in claim 7, wherein therespective moduli of elasticity of the stem and the bung are adapted toadjust the relative movement of the stem and the bung when the plungeris pressed.
 9. A hypodermic syringe as claimed in any preceding claimwherein the stem and the bung are permanently connected together.
 10. Ahypodermic syringe as claimed in claim 9, wherein the stem and the bungare of unitary construction.
 11. A hypodermic syringe as claimed inclaim 10, wherein the stem and the bung are made of similar elastomericmaterial differentially treated to make the stem relatively rigid andthe bung relatively elastic.
 12. A hypodermic syringe as claimed inclaim 11, wherein the stem and the bung are made of synthetic plasticsmaterial.
 13. A hypodermic syringe as claimed in any preceding claimcomprising a sleeve between the stem and the bung.
 14. A hypodermicsyringe as claimed in any preceding claim wherein the barrel istransparent.
 15. A hypodermic syringe as claimed in claim 14, whereinthe barrel comprises a tube of clear synthetic plastics material.
 16. Ahypodermic syringe as claimed in any preceding claim wherein the barrelcontains a metered amount of said injectant.
 17. A hypodermic syringe asclaimed in claim 16 wherein the barrel is labeled to indicate theinjectant and the amount.
 18. A hypodermic syringe as claimed in claim16 or claim 17, wherein the injectant is a local anaesthetic agent. 19.A hypodermic syringe as claimed in any preceding claim comprising a sealclosing the forward end of the barrel, wherein said connecting meansconnects said needle to the barrel in a first position and the needle isadjustable thereon to move to a second position in which it pierces saidseal.
 20. A hypodermic syringe as claimed in claim 19, wherein theneedle has a tip at its forward end for hypodermic intromission and atail at its rearward end for piercing said seal.
 21. A hypodermicsyringe as claimed in claim 19 or claim 20, wherein the barrel, theconnecting means, the bung, the stem, the seal and the needle are afteruse disposable connected together.
 22. A hypodermic syringe as claimedin any preceding claim wherein the needle is permanently connected tothe barrel.
 23. A method of making a hypodermic syringe according to anypreceding claim by adaptation of a conventional cartridge for dentallocal anaesthetic, which method comprises securing said connection meansto the forward end of the cartridge and securing to the rearward end ofthe cartridge an assembly comprising said bung and said stem and a coverproviding a guideway for the stem.